1. Field of the Invention
The present invention relates to an image forming apparatus and method for correcting a color registration. More particularly, the present invention relates to an image forming apparatus and method for correcting a color registration according to an intermediate transfer belt condition.
2. Description of the Related Art
Electrophotographic printers such as color laser printers comprise four color photoconductive drums Dy, Dc, Dm, and Dk corresponding to four colors, such as, yellow, cyan, magenta, and black, respectively. A laser scan unit scans the four photoconductive drums Dy, Dc, Dm, and Dk with laser beams to form desired electrostatic latent images. A developing unit develops the electrostatic latent images using color developers. A transfer belt transfers a completed color image, which is obtained by overlapping the images developed on the photoconductive drums Dy, Dc, Dm, and Dk with one another to paper.
A single color image may be obtained by developing color images formed on the four photoconductive drums Dy, Dc, Dm, and Dk using color developers, overlapping the color images at the same position of the transfer belt, and transferring a completed color image to paper.
In order to accurately overlap the four-color images at the same position of the transfer belt and obtain a desired color image, four colors have to match in a transfer beginning position and a transfer ending position. If the images formed on the four photoconductive drums Dy, Dc, Dm, and Dk are all vividly developed, but are out of registration when being transferred to the transfer belt, a clear color image cannot be obtained.
Therefore, it is important to take into account a travel velocity of the transfer belt and match points of scanning time when the photoconductive drums Dy, Dc, Dm, and Dk are exposed to laser beams by the laser scan unit. The scanning time matching to overlap a plurality of color images accurately is referred to as “color registration”.
If the scanning times are matched according to an initially set value, an error may still occur in the color registration as a printing operation progresses. The error occurring in the color registration may be caused by various reasons such as an error caused by inaccurate arrangements of a color developing unit, error occurring in the manufacturing of an optical lens, and error occurring in the driven transfer belt. Additional errors may occur if the printer has a serial structure composed of a plurality of developing units.
FIG. 1 is a view illustrating a system that adopts a conventional method for correcting color registration. Referring to FIG. 1, the system comprises a color registration sensor 10, intermediate transfer belt 20, controller 30, and laser scan unit 40.
The color registration sensor 10 comprises a light emitting unit 13 and a light receiving unit 17. The light emitting unit 13 emits a constant level of light toward the intermediate transfer belt 20. The light receiving unit 17 detects a reflection pattern of light reflected from the intermediate transfer belt 20.
The intermediate transfer belt 20 has a certain mark that is formed by the laser scanning unit 40, which is necessary to correct a color registration. The controller 30 receives the reflection pattern from the light receiving unit 17 and corrects the color registration based on the reflection pattern.
The laser scan unit 40 irradiates laser beams and selectively exposes charged photoconductive media to laser beams, thereby forming electrostatic latent images on surfaces of the photoconductive media, and simultaneously forms a certain mark on the intermediate transfer belt 20 by scanning the intermediate transfer belt 20.
FIG. 2 is a flowchart illustrating a conventional method for correcting a color registration. Referring to FIGS. 1 and 2, the laser scan unit 40 forms a certain mark on the intermediate transfer belt 20 that is required to correct a color registration at operation S200. The certain mark may use various types of patterns including a total reflection or irregular reflection pattern.
Next, the light emitting unit 13 of the color registration sensor 10 emits light toward the intermediate transfer belt 20 at operation S210. The light receiving unit 17 of the color registration sensor 10 detects a reflection pattern. The reflection pattern is estimated as a voltage level by the light receiving unit 17 at operation S220.
The light receiving unit 17 transmits the estimated voltage level to the controller 30 and the controller 30 determines whether it is possible to correct a color registration based on the transmitted control level (voltage level) at operation S230.
If it is possible to correct a color registration, the controller 30 stores location information regarding the mark formed on the intermediate transfer belt 20 based on the voltage level at operation S240. According to the location information, an X offset, Y offset, width error, and skew are calculated.
The X offset indicates an error occurring in a direction of scanning, which is a direction where the color registration sensor 10 scans. The Y offset indicates an error occurring in a direction where the intermediate transfer belt 20 advances. The width error indicates an error occurring due to a difference between horizontal widths in an image area, and the skew indicates an error occurring due to tilted scanning lines.
The controller 30 corrects the color registration based on the above calculated data at operation S260. The controller 30 also controls the scanning times of the laser scan unit 40 based on the corrected color registration and performs a printing operation at operation S270.
However, if the intermediate transfer belt 20 has an uneven surface or a partially abnormal section appears due to the change in the surface of the intermediate transfer belt 20, it is impossible to correct the color registration accurately.
That is, according to the conventional method for correcting the color registration, the color registration is corrected regardless of the surface condition of the intermediate transfer belt 20. Since the color registration is corrected, print positions of respective color images are not accurately controlled due to the problem of the intermediate transfer belt surface. Accordingly, quality of images printed on paper deteriorates.